Method and installation for the thermal hydrolysis of sludge

ABSTRACT

The invention relates to a method for the thermal hydrolysis of sludge characterized in that it is carried out in at least two reactors operating in parallel, in each of which the sludge undergoes a complete thermal hydrolysis cycle. Said cycle comprises different stages consisting in supplying the reactor with the sludge, injecting live stream in order to bring said sludge to a pressure P and a temperature T at which hydrolysis can occur, maintaining the sludge at said pressure P and said temperature T for a certain amount of time, suddenly bringing said sludge to the atmospheric pressure by releasing the flash steam and emptying said reactor of the hydrolysed sludge. Furthermore, the inventive method consists in time shifting the cycle of one reactor to the other in order to use the flash steam produced from one reactor to inject it into the other reactor.

This application is a U.S. National Stage application of PCT ApplicationNo. PCT/FR02/00577, with an international filing date of Feb. 14, 2002.Applicant claims priority based on French application serial no. 0102038 filed Feb. 14, 2001.

This invention concerns the field of treatment of sludge which isheavily loaded with fermentable organic material and especially sludgeoriginating from decontamination processes of urban or industrial wastewater, production processes for potable water from surface orsubterranean water, or even sludge originating from sewerage mainscleaning operations.

Currently, part of the sludge produced by purification stations isimproved in the agricultural industry, whereas another part is stored inoverflow. Since production of this sludge is becoming more and moreimportant, it is necessary for the latter not to present any risk to theenvironment and human health. In fact, this sludge contains germswhereof certain are pathogens (coliform, salmonella bacteria, helmintheggs, . . . ). Also, they are highly fermentable and are the basis ofgas production (amines, hydrogen sulphide, mercaptans) which causeolfactive pollution.

These considerations explain the necessity of using, on theabovementioned products for treatment, at least one stabilisation stagefor sludge creating a sludge which no longer evolves, or at the least,evolving slowly, as much on the biological scale as on thephysico-chemical scale. Another preoccupation is the willingness toreduce the volume of this sludge.

Different processes have been proposed in the prior art for treatingthis sludge. These processes can be classed essentially as follows:

-   -   aerobic digestion,    -   anaerobic digestion,    -   chemical processing,    -   thermal processing.

It is the last type of treatment to which this invention relates.

Thermal hydrolysis of sludge consists of treating it at high temperatureunder pressure so as to hygienise it (that is to very strongly breakdown their micro-organism content) and to transform the organic matterwhich it contains into soluble biodegradable DCO (alcohols, aldehydes,volatile fatty acids).

The oldest technique of thermal hydrolysis of sludge was made availableby Portheous at the beginning of the century. According to thistechnique, which makes use of a certain number of reactors underpressure, functioning in sequence, the sludge is pumped into a firstreactor wherein steam produced by a boiler is injected until temperatureof around 180° C. is reached for a pressure of 15 bar. The temperatureis then maintained for 30 minutes, then the sludge is evacuated underits own pressure via a heat exchanger. This heat exchanger is utilisedto recuperate the heat of the sludge exiting from the reactor and toreheat the sludge to be treated entering a second reactor.

This process does, however, have the disadvantage of being discontinuousand yielding low productivity.

In the fifties, FARRER put forward a continuous and automatic process ofthermal hydrolysis of sludge. According to this process, the thicksludge undergo disintegration, before being pumped under pressure (20 to35 bar) to then be introduced into a so-called primary exchanger whichhas a double purpose: preheating the incoming sludge to around 180° C.,and cooling the sludge leaving the thermal treatment. The preheatedsludge then passes into a second heat exchanger where it is brought to atemperature higher than 200° C. The calorific contribution is cause byhot water produced by a boiler. The sludge is then conveyed to thereactor itself where treatment takes place over a given time and at agiven temperature. The sludge is then returned to the primary exchanger.At the end of the cycle the sludge is brought to atmospheric pressure bya discharge system.

This type of process has the disadvantage of generating nauseous odours,requiring the use of materials which quickly foul and thus impliesconsiderable maintenance. In addition, considerable capital is requiredand is reserved for major installations capable of arranging flow chainsbetween 20 and 50 m³/h to ensure continuous discharge of the sludge and,accordingly, to prevent the destructive phenomenon of cavitation andconstruct large-scale specially profiled passage exchanges.

WO 96/09882 describes a process functioning in batch and utilising fourreactors installed in series. According to this discontinuous process,the sludge is pumped into the different reactors, whereof the firstthree are heated, where it is treated. The sludge from the thirdreactor, heated by injection of live steam, and optionally from thefourth reactor, is relieved of pressure suddenly (flash) to produce theflash steam. The flash steam coming from the third reactor is sent on tothe second reactor. The flash steam coming form the fourth reactor issent on to the first reactor.

The disadvantage of this technique is being able to function onlydiscontinuously, that is, being able to treat only successive lots ofsludge. The production of live steam necessary for implementing thistechnique also cannot be continuous. The sludge passes through all thereactors during treatment. This implies the utilisation of materialssuch as valves, subjected to difficult operating conditions.

The object of the present invention is to propose a novel process fortreatment by thermal hydrolysis of sludge which does not have thedisadvantages of the prior art.

In particular, an object of the present invention is to describe such aprocess capable of implementing production and continuous use of livesteam.

Yet another object of the present invention is to divulge the type ofprocess which also enables continuous treatment of the sludge.

Another object of the present invention is to carry out the essentialfunction of the reactions concerning a lot of sludge in a single reactorso as to minimise the transfer of sludge.

An object of the present invention is also to enable thermal treatmentof previously dehydrated sludge.

Also, an object of the present invention is to propose an installationfor implementing such a process.

These different objects, along with others which will emergehereinbelow, are attained due to the invention which relates to aprocess for thermal hydrolysis of sludge, characterised in that it isfed into at least two reactors functioning in parallel in each of whichthe sludge undergoes a complete cycle of thermal hydrolysis, said cyclecomprising the steps consisting of supplying-said sludge to saidreactor, injecting live steam to bring it to a pressure P and atemperature T allowing hydrolysis, maintaining it at said pressure P andat said temperature T for a certain time, suddenly bringing said sludgeto atmospheric pressure by releasing the flash steam, and emptying saidreactor of said hydrolysed sludge, and in that it consists of shiftingsaid cycle in time from one reactor to the other to utilise the flashsteam produced from a reactor to inject it into the other reactor.

Such a process has the advantage of implementing a complete hydrolysiscycle of the sludge in each reactor and, by using at least two reactorswhose operating cycle is shifted, benefiting from the flash steamproduced in one of the reactors for supplying the other reactor withsteam.

By obviating the necessity of transmitting the sludge from one reactorto the other to carry out the different steps of thermal hydrolysis, theprocess simplifies the installations necessary for putting the processto use, diminish the fouling rate of these installations, and minimisethe odours produced as the sludge passes from one reactor to the other.

According to a preferred variant of the invention, the process proposedis carried out in at least three reactors functioning in parallel.

In this case, the live steam is injected continuously alternatively insaid three reactors.

Such an aspect of the process allows the live steam to be produced froma boiler continuously, without the concern of distributing the steamdiscontinuously.

Shifting the cycles starts of thermal hydrolysis from one reactor to theother will be more or less long.

Nevertheless, according to an interesting variant of the process, saidshifting will be calculated such that the process comprises three phases

-   -   a first phase wherein:        -   sludge is fed to a first reactor and flash steam coming from            a third reactor is injected into said first reactor,        -   live steam is injected into a second reactor containing            sludge,        -   a third reactor containing sludge is kept at hydrolysis            temperature and pressure then relieved of pressure, with the            released flash steam being fed to said first reactor, just            before being emptied;    -   a second phase wherein:        -   live steam is injected into said first reactor,        -   the second reactor is kept at hydrolysis temperature and            pressure then relieved of pressure, with the released flash            steam being fed inside said third reactor, just before being            emptied,        -   sludge is supplied to said third reactor and flash steam            coming from the second reactor is injected into said third            reactor;    -   a third phase wherein:        -   the first reactor is maintained at hydrolysis temperature            and pressure then brought to the relieved pressure, the            released flash steam being fed to said second reactor, just            before being emptied,        -   sludge is supplied to said second reactor and flash steam            coming from the first reactor is injected into said second            reactor,        -   live steam is injected into said third reactor.

According to this variant of the process, the live steam can begenerated continuously and distributed alternatively in the first,second and third reactors. The utilization of the flash steam producedby a sudden drop in the contents of the reactors is optimized. Suchcontinuous supply uses a boiler of simple design and a set ofslow-release valves for in turn distributing the steam produced in theboiler for supply.

According to another interesting variant of the invention, the processis put to use in more than three reactors functioning in parallel withadapted cycle times so that the live steam can be injected continuouslyalternatively into said reactors, the sludge being injected continuouslyalternatively into the reactors and extracted continuously alternativelyfrom the reactors.

Such a variant combines the advantages resulting from the continuousfeed of live steam and the continuous feed of the sludge to be treated.

The temperature and the pressure used inside the reactors to drive thethermal hydrolysis of the sludge may vary. Preferably, this pressure isbetween 10 bar 20 bar and this temperature is between 130° C. and 200 °C.

The duration of the complete cycles of thermal hydrolysis in eachreactor can vary in particular according to the nature of the sludge andthe intensity of the treatment it is to undergo. Each cycle combines thesteps of:

-   -   feeding a reactor with sludge;    -   injecting flash steam;    -   injecting live steam;    -   retention;    -   pressure reduction; and,    -   evacuation of the sludge from the reactor.

According to a preferred variant, the duration of each cycle is between100 nm and 360 nm. In a most preferred way, the duration of this cyclewill be between around 150 nm and around 160 nm when three reactors areused.

The shifting in time of the hydrolysis cycle from one reactor to theother can also vary. It will be preferably selected to allow Continuousfeed of the steam successively into the reactors.

According to another particularly interesting aspect of the invention,the proposed process preferably comprises a mesophilic on thermophilicdigestion step of the hydrolysed sludge resulting in the production ofbiogas, said biogas being used to generate at least part of the livesteam necessary for thermal hydrolysis carried into effect in said atleast two reactors functioning in parallel. The digestion step can alsobe thermophilic aerobic.

Such a combination of the process with mesophilic or thermophilicdigestion can boost yields of the digestion step, and reduce emissionsof nauseating odours, and optimize the production of biogas. Thiscombination also helps reduce the quantity of residual sludge and obtainsludge which is perfectly stabilized and hygienised.

According to a variant of the process, said digestion step is coupledwith thermal hydrolysis, the sludge generated by said digestion beingmixed, in all or part, with the excess sludge and hydrolysed again. Suchcoupling achieves major mineralization of the sludge and thusconsiderably reduces its volume.

Also, according to a preferred variant of the invention, the processcomprises a step consisting of thermocompressing said flash steam bysaid live steam and thus optimizing the cycle time of hydrolysis btperforming the heating phase in one step instead of two.

The digested sludge can also, at least in part, be incinerated oroxidised via humidity.

The invention also relates to an installation for implementing theprocess described hereinabove, characterised in that it comprises atleast two hydrolysis reactors mounted in parallel, means for supplyingsludge to be hydrolysed in each of said reactors, means for evacuatinghydrolysed sludge from each of said reactors, means for injecting thelive steam alternatively into each of said reactors and means forconveying the flash steam coming from each reactor to the other reactor.

Preferably, said installation also includes at least one digester.

According to a variant the installation also comprises incinerationmeans or humid oxidation means of all or part of the digested sludge.

The invention, as well as the different advantages it brings, will bebetter understood from the following description of an embodiment of theinvention given by way of reference to the diagrams, in which

FIG. 1 illustrates a first embodiment of an installation for utilisingthe process according to the present invention;

FIG. 2 illustrates a block diagram of the operation of the installationillustrated in FIG. 1;

FIG. 3 illustrates a second embodiment of an installation for utilisingthe process according to the present invention.

With reference to FIG. 1, the installation illustrated comprises threehydrolysis reactors 1, 2, 3, mounted in parallel. Each reactor 1, 2, 3is provided with means for conveying sludge, respectively 4, 5, 6 to behydrolysed and means for evacuation of the sludge, respectively 7, 8, 9.

The installation also comprises injection means 10, 11, 12, forinjecting the live steam alternatively in each of said reactors 1, 2, 3and routing means 13, 14, 15 for the flash steam coming from eachreactor 1, 2, 3 to the other reactor. The means for injecting live steamare connected to a boiler (not illustrated here).

Feeding the reactors with sludge to be treated, directing live steam,injecting flash steam, and emptying the reactors, alternatively in eachreactor is organised by a control element 16 linked especially to allthe valves of the plant (for the sake of clarity of FIG. 1, these linksare not shown).

The operation of the installation shown in FIG. 1 will now be explainedwith reference to FIG. 2.

Each thermal hydrolysis cycle is completed over a period of 160 nm (2h40) and comprises the following steps of:

-   -   feeding a reactor with sludge: 15 nm    -   injecting flash steam coming from another reactor: 20 nm    -   injecting live steam: 60 nm    -   retention: 30 nm    -   pressure release resulting in production of flash steam: 20 nm    -   emptying: 15 nm.

The accumulated times relative to these different steps are indicatedfor the reactor 1 in FIG. 2 (these times are given indicatively only).

Three operating phases of the installation can be observed, each ofthese phases being symbolised in FIG. 2 by double braces.

During the first phase (phase 1), the reactor 1 is fed with sludge to betreated, and flash steam coming from the reactor 3 (for which thethermal hydrolysis cycle has already commenced, as indicated by thehorizontal dotted lines) is injected therein. During this phase 1 thelive steam coming from the boiler is injected into the reactor 2 (forwhich the thermal hydrolysis cycle has already commenced, as indicatedby the horizontal dotted lines). In the third reactor the retention steptakes place and the pressure inside the reactor is rapidly brought downto the reduced pressure to obtain the flash steam which, as alreadyindicated, is redirected to the reactor 1. The first phase lasts 35 nm.

In the course of the second phase the live steam is injected into thereactor 1, the contents of the reactor 2 are kept under pressure thensupplied to reduced the pressure, the flash steam thus emitted beingdirected to the third reactor, the latter having previously been emptiedand then resupplied with sludge to be treated. The second phase lasts 60nm.

During the third phase, the contents of the reactor 1 are kept underpressure then brought to atmospheric pressure, the flash steam thusemitted being directed to the second reactor, the latter havingpreviously been emptied and then resupplied with sludge to be treatedand live steam is injected in the third reactor. The third phase lasts65 mn.

It will be noticed that between emptying a reactor and resupplying it, arest time of around 5 mn to 10 mn is observed.

As is clearly evident from FIG. 2, the process according to the presentinvention has the advantage of enabling continuous distribution of thelive steam coming from the boiler alternatively in the reactors 1, 2 and3.

With reference to FIG. 3, the installation illustrated in FIG. 1 anddesignated in its entirety by reference A can be combined with amesophilic on thermophilic digestion unit designated by reference B andwith a dehydration unit for sludge designated by reference C.

According to this embodiment, the excess sludge to be treated isconveyed via channel 20 to the dehydration unit C, then directed to thethermal hydrolysis unit such as described with reference to FIG. 1. Onceit is hydrolysed, this sludge is routed to the mesophilic digester B (itis noted that in other embodiments this digester could also function inthermophilic mode). The biogas obtained during digestion is recovered at22 to heat a boiler 23 which supplies, via the channel 24, the livesteam necessary for operating the unit A. The sludge coming from thedigester is then redirected to the top of the plant to the dehydrationunit C. The strongly mineralises sludge residue is recovered at 25. Thisresidue is fully hygienised and deodorized. Its volume, relative to theinitial volume, is very low.

The resulting digested sludge can be dehydrated again to then beimproved in agriculture, for example.

This sludge can also be sent to a thermal destruction process such ashumid oxidation or such as incineration. The dimensions of theincineration ovens used in the latter case can be smaller than the sizeof those which would have to be used if the sludge in question had notundergone thermal hydrolysis.

The aim of the embodiments of the invention described here is not toreduce the scope of the latter. Numerous modifications can be addedwithout departing from its scope. In particular it is noted that, inanother embodiment, more than three reactors mounted in parallel can beused to enable not only continuous feed of live steam but alsocontinuous supply and extraction of sludge. The duration of the variousphases could also be selected in different ways.

1. A multi-phase method of treating sludge in a series of reactors,comprising: In a first phase: directing sludge to a first reactor;directing live steam to a second reactor; retaining sludge in a thirdreactor, reducing the pressure in the third reactor and releasing flashsteam from the third reactor to the first reactor; In a second phase:directing sludge to the third reactor; directing live steam to the firstreactor; retaining sludge in the second reactor, reducing the pressurein the second reactor and releasing flash steam from the second reactorto the third reactor; and In a third phase: directing sludge to thesecond reactor; directing live steam to the third reactor; and retainingsludge in the first reactor, reducing the pressure in the first reactorand releasing flash steam-from the first reactor to the second reactor.2. The method of claim 1 wherein the sludge is retained at apredetermined pressure and a predetermined temperature.
 3. The method ofclaim 2 wherein the predetermined temperature is about 130° C.-200° C.4. The method of claim 2 wherein the predetermined pressure is about 10bar-20 bar.
 5. The method of claim 1 wherein the sludge is maintained ineach reactor for about 100 minutes-360 minutes.
 6. The method of claim 1further comprising digesting the sludge to produce a biogas and adigested sludge and using said biogas to generate at least a part of thelive steam.
 7. The method of claim 6 wherein digesting the sludgeincludes subjecting the sludge to an aerobic treatment.
 8. The method ofclaim 6 wherein digesting the sludge comprises subjecting the sludge tomesophilic digestion.
 9. The method of claim 6 wherein digesting thesludge comprises subjecting the sludge to thermophilic digestion. 10.The method of claim 6 wherein digesting the sludge comprises subjectingthe sludge to thermophilic aerobic digestion.
 11. The method of claim 6further comprising mixing at least a part of the digested sludge with atleast a part of the sludge directed to one of the reactors.
 12. Themethod of claim 6 further comprising directing at least part of thedigested sludge to an incineration unit.
 13. The method of claim 6further comprising directing at least part of the digested sludge to anoxidation unit.
 14. The method of claim 1 wherein in the first phase,sludge is removed from the third reactor, in the second phase sludge isremoved from the second reactor, and in the third phase sludge isremoved from the first reactor.
 15. The method of claim 1 wherein duringeach phase, flash steam is directed from only one of the reactors toeither of the other two reactors.
 16. The method of claim 1 furthercomprising thermo-compressing the flash steam emitted from at least onereactor with the live steam.
 17. A method of treating sludge in a seriesof reactors in a multiphase process comprising: in a first phasedirecting sludge into a first reactor, directing live steam into asecond reactor that received sludge during a previous phase, anddirecting flash steam from only a third reactor to the first reactor anddirecting treated sludge from the third reactor; and repeating the firstphase method for at least two additional phases where sludge issequentially directed into the second and third reactors and whereinlive steam is sequentially directed into the third and first reactors,and wherein flash steam is sequentially directed from the first andsecond reactors to the second and third reactors and treated sludge issequentially removed from the first and third reactors.
 18. The methodof claim 17 wherein during each phase sludge is retained in at least onereactor and subjected to a temperature of about 130° C. to 200° C. for aselected time period.
 19. The method of claim 18 wherein the selectedtime period is about 100 minutes to 360 minutes.
 20. The method of claim17 wherein during each of the three phases, sludge within one of thereactors is removed and wherein over the course of the three phases,sludge is removed from the first, second and third reactors.
 21. Themethod of claim 17 wherein the sludge retained in at least one reactorduring each phase of the process is subject to a pressure of about 10bar-20 bar.